I'll take a crack at some of these special abilities:
Spit (Ex): Once per day, a marsh dragon can spit at one creature as a ranged touch attack. A successful spit attack deals 1d6 points of acid damage (Fort/Ref save DCx for half ?) and infects the target with marsh fever (no save because the spit is highly infectious).
Breath Weapon: The marsh dragon's breath weapon is a cone of infectious vapor. A creature caught in the area of the breath takes the hit point damage indicated due to sudden exposure to numerous pathogens (Ref save half). In addition, it must make a Fort save or contract marsh fever.
Marsh Fever: This is a supernatural inhaled disease with a 1 day incubation period which always originates from a marsh dragon. Each day, the victim must make a Fort save (of the same DC as the dragon's breath weapon) or take 1d6 hit points of damage per age category of the originating dragon. This disease can be cured only by the spells remove disease, heal, or mass heal. The caster must succeed at a caster level check with DC equal to 10 + the originating dragon's age category.
I thought about making this contagious, but maybe that would be too devastating if an explorer happened to come by after getting infected. Anyway, the original mechanics seemed a bit clunky, but I tried to keep the spirit of it.